Color imaging devices, such as color copiers, color printers, color printing presses and other similar color imaging devices often exhibit visible color variations in their printed output. These visible variations may occur within a single page (spatial page variations), and also from page to page (page-to-page variations), even though the input to the imaging device is identical for each page. This phenomenon is often referred to as “engine drift,” or “engine instability,” and may result from variations in manufacturing tolerances or aging of the imaging device, time-dependent variations in the colorants used to form the image (e.g., toner or ink), and variations in the ambient environment of the device.
Although engine drift and engine instability have been recognized for some time, there has not been an objective technique to characterize this phenomenon that correlates well with the visually perceived phenomenon. Indeed, previously known techniques for characterizing engine drift have often been based on density measurements. In one such previously known technique, a color imaging device is used to print a test pattern that includes test patches, which are then measured using a densitometer or other similar device that provides measurements of reflected density. After obtaining a large number of measurements (i.e., from multiple locations within a page and from multiple pages) the measured density values are averaged, and a standard deviation measurement is calculated from the measured data.
Although such previously known density-based techniques may be used to characterize engine drift, the techniques are not useful indicators of human perception of engine drift. In particular, density measurements do not correlate well with human perception of color, and density-based indicators of color variation do not correlate well with human perception of color differences. As a result, previously known density-based techniques for characterizing engine drift do not bear a close relationship to how a human observer would perceive such engine drift.
It therefore would be desirable to provide methods and apparatus for objectively characterizing color variations in a color output device.
It further would be desirable to provide methods and apparatus for characterizing color variations in a color output device in a manner that corresponds to human perception of such variations.